当我询问“难度”后，我的程序无法继续应用程序

Question

我试图根据代码是什么条件，但是我很难确定我的条件是什么。

用户输入是否要进行简单或困难的数学测验以及他们想回答的问题数量。然后，该应用程序一次显示一个问题，提示用户输入答案并确认答案是否正确。该应用程序应包括用于简单和困难数学测验的单独方法。 simple（）方法应显示其他问题。 hard（）方法应显示乘法问题。应为测验问题生成随机数。

import java.util.Scanner;

public class Project03 {

    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        System.out.print("Difficulty?");
        String difficulty = keyboard.nextLine();


        int randomNumber1 =  (int)(20 * Math.random()) + 1;
        int randomNumber2 =  (int)(20 * Math.random()) + 1;
        int randomNumberAdd = randomNumber1 + randomNumber2;
        int randomNumberMul = randomNumber1 * randomNumber2;
        int correct = 0;

        if (difficulty == "Simple"){

            System.out.print(randomNumber1 + " + " + randomNumber2 + " = ");
            int GuessRandomNumberAdd = keyboard.nextInt();

            if (GuessRandomNumberAdd == randomNumber1 + randomNumber2) {
                System.out.println("Correct!");
                correct++;
            }else {
                System.out.println("Wrong!");
                System.out.println("The correct answer is " + randomNumberAdd);
            }
        }



        if (difficulty == "Difficult") {  


            System.out.print(randomNumber1 + " * " + randomNumber2 + " = ");    
            int GuessRandomNumberMul = keyboard.nextInt();

            if (GuessRandomNumberMul == randomNumber1 * randomNumber2) {
                System.out.println("Correct!");
                correct++;
            }else{
                System.out.println("Wrong!");
                System.out.println("The correct answer is " + randomNumberMul);
            }
        }
    }
}